基于APD阵列的3D激光雷达

The following paper will elaborate on the existing Lincoln efforts dedicated to 3-D laser radar technologies and applications.
GEIGER-MODE INTEGRATED APD DETECTOR ARRAY DEVELOPMENT
far into saturation. The current pulse out of the device becomes very reproducible and can be used to trigger digital timing circuitry directly. Effectively, the device becomes sensitive to a single photon and the output is noiseless, except for fundamental shot noise. The saturation of the device from more than one photoelectron being generated in the absorption region can be addressed by limiting the signal level to an average of less than a photoelectron per laser pulse and looking statistically at the number of detections per number of laser pulses. This represents the most sensitive form of optical measurement in which every photon is counted.
t First Photon Arrival Time Figure 1: Geiger-mode APD arrays bonded to CMOS timing circuitry.
Once the APD "fires", or breaks down due to the runaway avalanche process, a well-defined voltage pulse is generated that can directly toggle CMOS digital circuitry. As can be seen from Figure 1, each pixel of the APD array has an associated timing register. This is achieved by bonding the APD array to an array of CMOS timing circuitry. A block diagram of the timing circuit for each pixel is shown in Figure 2. The counter in the timing circuit consists of a 15-bit register with positive feedback. This configuration will count through a sequence of 215-1 psuedo-random numbers. Since the exact sequence is known, the random number can be decoded during readout with a simple lookup table to produce a count. The advantage to this type of counter is that it can be realized with a minimum of circuitry. Two effective bits of timing resolution are added by recording the clock state and a 90-degree phase-shifted version of the clock. Thus, 0.5-ns timing resolution can be achieved with a 500-MHz clock. The sequence of events for a 3-D measurement then involves sending out a laser pulse to flood-illuminate the field of view of the detector. A short time before the backscattered return from the target arrives at the receiver the pixel clocks are turned on by broadcasting a common clock to each of the pixel counters. As the photons arrive and cause the APD’s to fire they latch and save the associated time as the count in the pixel timing register and the associated vernier bits. Finally, the digital values from each of the registers are read out in a serial fashion and the decoded time values are used to produce the 3-D image.
An overview of the silicon Geiger-mode detector array configuration and development has been reported in earlier publications.1-2 The basic configuration of the detector array is shown in Figure 1. The integrated focal plane consists of an array of Geiger-mode APD detectors epoxy bonded to a commensurate array of CMOS timing circuitry. Each hybrid pixel then contains a Geiger-mode APD and a timing circuit. The Geiger-mode APD can be thought of as a photodiode whose back bias voltage actually exceeds the level at which the detector will bread down. When photoelectrons are generated at the device junction due to the absorption of one or more signal photons (in practice, the devices have separate absorption and avalanche regions) they are accelerated in the electric field from the bias voltage. In the case of a "linear"-mode APD, the photoelectrons are accelerated enough to knock more electrons from the lattice and the total number of photoelectrons increases, resulting in gain. In a Geiger-mode device, the gain is so high that enough photoelectrons are generated to cause the device to breakdown and allow a large current to flow until any capacitance across the device is discharged. The advantage to this mode of operation is that variations in the gain from pixel to pixel or as a function of operating parameters or just due to statistical variations become irrelevant, since the device is driven
Avalanche-photodiode array
APD CMOS Timing Circuitry
hν
External Clock
100 µm
Timing-circuitry array
APD Active Area
Timing Register
Photon Arrival Time
VOUT
Threshold
* This research is sponsored by the U.S. Air Force under Contract F19628-00-C-0002. † The views expressed are those of the authors and do not reflect the official policy or position of the U.S. government. ‡ email: heinrichs@; phone: 781-981-7945; fax: 781-981-5069